The present invention generally pertains to communication signal processing apparatus and is particularly directed to detecting the presence of a predetermined recognition pattern in a communication signal and to acquiring a received communication signal at a given intermediate frequency and at a given phase when the communication signal was modulated onto a carrier signal.
Some communication signals include a predetermined recognition pattern including symbols that occur at a predetermined rate, which recognition pattern may be detected by a communication signal processor to recognize the presence of the communication signal.
In some communication signal processors, the presence of a recognition pattern is detected by processing in-phase (I) and quadrature-phase (Q) components that are extracted from the received communication signal. An example of such processing is described by Saulnier et al., "A VLSI Demodulator for Digital RF Network Applications: Theory and Results," IEEE Journal on Selected Areas in Communications, Vol. 8, No. 8, Oct. 1990, pp. 1500-1511.
Communication signals modulated onto a carrier signal are demodulated upon receipt by mixing the carrier signal with a local oscillator signal to acquire the communication signal at an intermediate frequency, which is further processed to provide the communication signal at a given baseband frequency.
Transmission disturbances, such as high Doppler rate shifts, specular multipath interference and ionosphere anomalies cause the frequency and the phase of a carrier signal to be offset during transmission, whereby the amount of the frequency offset and the amount of the phase offset must be determined and compensated for in order to acquire the received communication signal at the given intermediate frequency and at the given phase.
In some receivers, a mixer mixes the carrier signal with a local oscillator signal provided by a local oscillator at a local oscillator frequency to provide the communication signal at an intermediate frequency, a signal processor processes baseband I and Q components extracted from the communication signal at the intermediate frequency to compute the frequency offset and provides a frequency error signal having a value representative of the computed frequency offset, and the local oscillator responds to the frequency error signal by adjusting the frequency of the local oscillator signal in accordance with the computed frequency offset so that the mixer provides the received communication signal at the given intermediate frequency.
To compensate for the phase offset, the signal processor also processes the extracted I and Q components to compute the phase offset and provides a phase error signal having a value representative of the computed phase offset, and the local oscillator responds to the phase error signal by adjusting the phase of the local oscillator signal in accordance with the computed phase offset so that the mixer provides the communication signal at the given phase.
Various techniques for determining the frequency offset and the phase offset of a received communication signal by processing baseband I and Q components extracted from the received communication signal, are described by Natali, "AFC Tracking Algorithms," IEEE Transactions on Communications, Vol.Com-32, No. 8, Aug. 1984, pp. 935-947; Bellini et al., "Digital Frequency Estimation in Burst Mode QPSK Transmission," IEEE Transactions on Communications, Vol. 38, No. 7, July 1990, pp. 959-961; and Sollenberger and Chuang, "Low-Overhead Symbol Timing and Carrier Recovery for TDMA Portable Radio Systems," IEEE Transactions on Communications, Vol. 38, No. 10, October 1990, pp. 1886-1892.